1. Field of the Invention
The invention relates to alkoxy-crosslinking RTV1 silicone rubber mixtures based on polydiorganosiloxanes with dialkoxyorganosiloxy end groups, the organo radical being a substituted amidoalkyl radical.
2. Discussion of the Background
Single-component silicone rubber mixtures (RTV1) which can be stored if water is excluded and which vulcanize to give elastomers at room temperature when water is present are known. They generally comprise a polymeric, usually linear siloxane, a compound with crosslinking action, which must contain groups which are easily hydrolyzable, a plasticizer, which is usually a methyl-terminated polydimethylsiloxane, and optional additives such as curing accelerators, pigments, processing aids and fillers. The mixtures may vulcanize under acidic conditions, e.g. in the presence of acetoxysilanes, basic conditions, e.g. using aminosilanes, or neutral conditions, e.g. by means of compounds which have oximo or alkoxy groups. RTV1 systems which crosslink under neutral conditions are necessary especially when it is important that the elimination products produced during curing of the mixtures, should not affect the substrate, as for example in the jointing of concrete or metallic materials.
For formulating RTV1 materials, a polymer with OH groups is normally used. This procedure is used, for example, with acetate-, oxime- and amine-crosslinking mixtures. It is also possible to use hydroxyl-terminated polydiorganosiloxanes in the preparation of alkoxy-crosslinking RTV1 materials. EP 384 609 and EP 543 615, for example, describe methoxy-crosslinking materials, mixed with a large amount of filler, which are prepared using OH-terminated polydimethylsiloxanes. However, this method is successful only if the polymer, the plasticizer and the fillers (in this case a combination of precipitated and ground chalk) are initially mixed with one another, followed by the addition of methyltrimethoxysilane as a crosslinking agent, and a crosslinking catalyst. This initially gives a material which has an extemely high viscosity or is even to some extent elastomeric, and which can only be processed using specific mixing technologies. The mixing technology normally used for RTV1 materials, e.g., planetary mixers and dissolvers, cannot be used. In addition, this procedure is restricted to highly filled materials. It cannot be used for transparent systems filled only with fine-particle silica. It is known that mixing a polymer having an OH-end-group with silica without a crosslinking agent gives a very highly viscous paste, which stiffens rapidly to give a hard crumbly material. When a crosslinking agent is added to a mixture of this type, the firm consistency of the paste is lost. However, it is just such firm-consistency materials which are most frequently needed. In addition, it is not possible to control the premature crosslinking of materials prepared by the prior art methods just described.
For this reason, polymers with alkoxy end groups are used for alkoxy-crosslinking RTV1 systems. Their preparation is known, and generally takes place by reacting a hydroxyl-terminated polydiorganosiloxane of a certain viscosity with alkoxysilanes in the presence of catalysts (inter alia EP 137 883, EP 304 701, EP 559 045). During this condensation reaction(elimination of alcohol), the desired polymer is formed. The preparation of the mixture comprising the alkoxy-end-group polymer can then be prepared using the method described above. However, a disadvantage of all processes previously described is that an adequate extent of reaction of the OH-end-group polymers is achieved only when methoxysilanes are used. If the ethoxysilanes known hitherto are used for preparing the polymers, complete reaction of the OH groups is not achieved. For this reason, curing of the materials can occur as early as during the preparation of the RTV1 mixtures or during storage of the finished mixtures, i.e. the products obtained have inadequate stability when stored.
It is furthermore desirable, because of the toxicity of methanol, to be able to prepare alkoxy-crosslinking RTV1 materials which produce only non-hazardous elimination products, such as ethanol.